Recently, organic EL displays have attracted attention as the next-generation flat display panels. The organic EL displays have such advantages as self-luminescence, wide viewing angle, high-contrast image, thin profile, lightweight, and low power consumption.
Each of the organic EL devices that constitute an organic EL display generally includes a pixel electrode, a counter electrode, and an organic layer arranged between the pixel electrode and counter electrode. The organic layer is composed of a light-emitting layer containing fluorescent molecules, and an electron conductive thin film and a hole conductive which sandwich the light-emitting layer. Application of voltage between the pixel electrode and the counter electrode results in injection of electrons into the electron conductive thin film from the counter electrode and injection of holes into the hole conductive thin film from the pixel electrode, with the result that the electrons and holes are recombined in the light-emitting layer to cause luminescence.
In the manufacture of an organic EL display, the formation of an organic layer laminate is a critical process because the status of the organic layers has a great effect on the luminous efficiency and power consumption of the organic EL display. Methods of forming organic layers are broadly classified into two groups according to whether low-molecular or polymer material is chosen as organic material.
In the case of low-molecular organic materials, an organic layer laminate can be generally formed by vapor deposition. In this process, a substrate and a low-molecular material are placed in a vacuum chamber in the first place. After placing the chamber under vacuum, the low-molecular material is evaporated by electric resistance heating onto the substrate to form thereon an organic layer. By repeating this vapor deposition process for each different organic material, an organic layer laminate can be formed.
In the case of polymer organic materials, on the other hand, an organic layer laminate can be formed by coating methods. A solution of polymer organic material is applied (printed) by inkjet printing or the like on necessary areas and dried to form an organic layer. By repeating the cycle of coating and drying for each different organic material, an organic layer laminate can be formed (see, e.g., Patent Literatures 1 to 5).
In recent years large-screen displays have been required. The development of large-screen organic EL displays is also underway. As described above, the formation methods of organic layers are broadly classified into two groups according to the type of organic material used. In terms of manufacture of large-screen displays, however, coating methods are advantageous over vapor deposition for the reasons described below.
In the case where vapor deposition is employed upon formation of organic layers, because the type of organic material which constitutes a light-emitting layer varies from one luminescent color to another, different organic materials need to be deposited for different luminescent colors. In order to deposit organic materials in place according to the luminescent color, a metallic mask is generally used to mask those areas on which organic materials are not to be deposited. However, the larger the size of an organic EL display to be manufactured, the more it becomes difficult for the metallic mask to achieve precise masking due, for example, to its dimensional accuracy or deformation caused by thermal expansion during vapor deposition. On the other hand, in the case where organic layers are formed by coating methods, it is possible to apply a solution of organic material in proper areas without having to mask unwanted areas with a metallic mask. Thus, since coating methods eliminate the need to use a mask, they are advantageous methods of forming organic layers during the manufacture of a large-screen display.
When forming organic layers by coating methods such as inkjet printing, a solution of organic material needs to be applied in such a way that a uniform thickness is achieved across the entire surface of the substrate. In some cases, it is difficult to apply a solution of organic material at a uniform thickness across the entire surface of the substrate. Inkjet printing, for example, may not be able to achieve uniform application of coating solution over the substrate surface due, for example, to unstable ejection from nozzles or nozzle clogging. Moreover, when foreign material is present on the surface to be coated, there is a possibility of failing to uniformly apply the solution at a desired thickness, even when ejection from nozzles is stable. In such cases, a uniform thickness cannot be ensured over the substrate surface, resulting in film thickness variation over the substrate surface. In some cases, uncoated areas may occur. The presence of such defects in the organic layers causes brightness variation and/or creates non-luminescent portions when the display is illuminated. This eventually leads to low manufacturing yields of organic EL displays.
In order to solve the foregoing problem, methods of repairing defects in organic layers have been proposed (see, e.g., Patent Literature 6). Patent Literature 6 discloses a method of repairing defects in organic layers which are formed by inkjet printing. In this method, organic layers are checked for a defect, and the detected defect is dissolved away by application of a solvent. Subsequently, a solution of organic material is selectively applied to the region from which the defect has been dissolved away, to reproduce an organic layer therein.
The method disclosed by Patent Literature 6, however, has a drawback that it cannot repair those defects whose thickness is greater than that of the surrounding organic layer (convex defect; see FIG. 3C). That is, a defect to be repaired with the method disclosed by Patent Literature 6 should be thinner than the surrounding normal areas so that the solvent applied does not spread over the normal areas.
Moreover, the method disclosed by Patent Literature 6 has a drawback that it is difficult to control the amount of coating solution to be re-applied to an appropriate level. That is, in the method disclosed by Patent literature 6, the amount of coating solution to be re-applied increases with increasing defect size, and decreases with decreasing defect size. Thus, because the amount of coating solution to be re-applied greatly varies depending on the defect size, it is difficult with this method to control the amount of coating solution to an appropriate level; therefore, complete defect repair may fail.
A technology that succeeded in overcoming such a problem is described in Patent Literature 7. Patent Literature 7 discloses a defect repair method which includes the steps of: removing a defect, a region contaminated with foreign material or the like; and re-applying a solution of organic material in the region from which the defect has been removed. Since a defect repair method such as that disclosed by Patent Literature 7 involves ablation of a defect, the method can repair those types of defects whose thickness is greater than that of the surrounding area. Moreover, with such a defect repair method, it is only necessary to determine the amount of coating solution to be re-applied according to the amount of the organic layer ablated, making it possible to control the amount of the coating solution to an appropriate level.